1 Field of the invention
The present invention relates to a positively chargeable developer for developing an electrostatically charged image, used in image forming methods such as electrophotography, electrostatic recording and electrostatio printing. More particularly, it relates to a positively chargeable developer used in direct or indirect electrophotographic development, that is strongly positively chargeable in a uniform state and can make visible a negatively electrostatically charged image, or make visible a positively electrostatically charged image by reversal development, to give a toner image with a high quality.
2. Related Background Art
A large number of methods have been conventionally known as eleotrophotography, as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publication No. 42-23910 (U.S. Pat. No. 3,666,363) and Japanese Patent Publication No. 43-24748 (U.S. Pat. No. 4,071,361), etc. In general, copies are obtained by forming an electrostatic latent image on a photosensitive member utilizing a photoconductive material and according to various means, subsequently developing the latent image by using a developer (hereinafter often "toner") to form it into a visible image, and transferring the toner image to a transfer medium such as paper as necessary, followed by fixing by the action of heat, pressure, a pressure hear fixing roller, or solvent vapor. In the case when the process comprises an image transfer step, there is commonly provided a step of removing the toner remaining on the photosensitive member.
As developing processes in which an eleotrostatic latent image is formed into a visible image by using a toner, known methods include the magnetic brush development as disclosed in U.S. Pat. No. 2,874,063, the cascade development as disclosed in U.S. Pat. No. 2,618,552, the powder cloud development as disclosed in U.S. Pat. No. 2,221,776, and the method in which a conductive magnetic toner is used, as disclosed in U.S. Pat. No. 3,909,258.
As toners used in these development processes, there has been hitherto used fine powder obtained by dispersing a dye and/or pigment in a natural or synthetic resin. For example, particles formed by finely grinding a binder resin such as polystyrene comprising a colorant dispersed therein, to have a size of about 1 to 30 .mu. are used as the toner. A toner incorporated with magnetic material particles such as magnetite is also used as the magnetic toner. In a system in which a two-component type developer is used, the toner is usually used by mixture with carrier particles such as glass beads and iron powder.
As a method of obtaining a developer capable of controlling positive electrostatic charge, a proposal is seen in Japanese Patent Publication No. 53-22447. This is a method in which a metal oxide powder treated with aminosilane is internally added in the toner particles. Detailed studies on this method made it clear that the method has some problems when, for example, colloidal silica, alumina, titanium dioxide, zinc oxide, iron oxides, .gamma.-ferrite or magnesium oxide was treated using various aminosilane compounds to obtain developers according to Examples described in the specification of this publication.
Most developers can not retain for a long time the properties desired to make reproduction with fidelity from latent images. They show desirable performance at the beginning, but can not retain the initial properties after continual use for a long period of time and becomes ineffective. For example, after a large number of copies are taken, fogging may occur and, in the copying of line images, black spots of toner may be generated around edges thereof, resulting also in a lowering of image density. As another problem, the development and transfer carried out under environmental conditions of high temperature and high humidity may result in a lowering of image density, and generation of black spots around line images, blank areas, and fog.
An aminosilane coupling agent conventionally used include, for example, the following: ##STR2##
The silica treated with these aminosilane coupling agents, when used as an additive of the toner, causes water absorption or moisture absorption as a result of the copying under conditions of high temperature and high humidity, resulting in a lowering of image density. It also brings about changes with time during long-term storage of the toner to cause image deterioration such as ground fogging or reveral fogging. When the aminosilane coupling agents as described above are used, a hydrophobic property imparting agent is usually used in combination so that the toner may have environmental resistance and triboelectric stability.
Japanese Unexamined Patent Publication No. 59-34539 (corresponding to U.S. Pat. No. 4,618,556) discloses a method in which a positively chargeable silica prescribed to have hydrophobic property within a specified range is used as a component of a positively chargeable toner. The positively chargeable silica subjected to hydrophobic treatment, however, has the problem that the reversal fogging becomes more serious with an increase in the hydrophobic property. In general, when a powder treated with an agent for making it hydrophobic is frictionally charged together with ion powder, it has the property of being negatively charged and tends to be strongly negatively chargeable with an increase in the hydrophobic property. Thus, this tends to be the cause of generating the reversal fogging in the state that no sufficient electrostatic charges have been imparted to the toner as in the initial state of copying.
Conventional agents for making the toner hydrophobic are exemplified by the following: Hexamethyldisilazane, trimethylsilane, trimethylchlorosilane, trimethylethoxysilane, dimethyldichlorosilane, methyltrichlorosilane, allyldimethylchlorosilane, allylphenyldichlorosilane, benzyldimethylchlorosilane, bromomethydimethylchlorosilane, .alpha.-chloroethyltrichlorosilane, .beta.-chloroethyltrichlorosilane, chloromethyl-dimethylchlorosilane, triorganosilylmercaptane, trimethylsilylmercaptane, triorganosilylacrylate, vinyldimethylacethoxysilane, and also dimethylethoxysilane, dimethyldimethoxysilane, diphenyldiethoxysilnane, hexamethyldisiloxane, 1,3-divinyltetramethyldisiloxane, 1,3-diphenyltetramethyldisiloxane, and a dimethylpolysiloxane having 2 to 12 siloxane units per molecule and containing in the unit positioned at the terminal a hydroxy group bonded to Si for each one. These are used alone or in the form of a mixture of two or more kinds.
Japanese Unexamined Patent Publication No. 59-201063 (corresponding to U.S. Pat. No. 4,568,625) discloses a method of obtaining the positively chargeable toner. This is a method in which fine powder of silica treated with silicone oil containing amine in the side chain is incorporated into a developer.
Detailed studies of this method confirmed that the image quality, density and fog were all in good states in commonly available copying machines even when a large number of copies were taken. When, however, the above toner is loaded into a high-speed copying machine (process speed: not less than 300 mm/s), a copying machine that can achieve multi-color development, and a digital copying machine having a low potential contrast of the image, the initial properties can not be retained after the continual use of the toner for a long period of time and the problem of the reversal fogging tends to occur.
Besides the fine silica powder, it is also known to add other additives in the developer. For example, Japanese Patent Publications No. 48-8136, No. 48-8141 and No. 51-1130 teach that a friction-reducing material such as polyvinylidene fluoride powder is used as an additive of the developer. Detailed studies on this method, however, revealed that the method effectively prevents the poorness in cleaning resistance and the melt-adhesion of toner to a drum, but on the other hand has the problems that the sharpness of toner images may be extremely lowered, the latent images on the photosensitive member tend to be deformed under conditions of high temperature and high humidity, and the stability in duration is unsatisfactory.
As a method of improving the cleaning performance, Japanese Unexamined Patent Publication No. 61-160760 (corresponding to U.S. Pat. No. 4,666,813 discloses a method in which fine particles of specific polyvinylidene fluoride are externally added to the toner. Detailed studies on this method obtained a good result particularly in relation to the cleaning performance. In some instances, however, a lowering of image density is seen when the toner is loaded in the high-speed copying machine and tested for long-term duration.